Infrared (IR) imaging devices, such as microbolometers or cooled IR imaging devices, comprise an array of IR-sensitive detectors forming a pixel array. To correct spatial non-uniformity between the pixels of such a pixel array, offset and gain correction is generally applied to each pixel signal (or “pixel value”) of a captured image before it is displayed. The offset and gain values are generated during an in-house preliminary calibration phase of the device using uniform emissive sources (black-bodies) at controlled temperatures, and stored by the imaging device. Such spatial non-uniformity varies not only in time but also as a function of the temperature of optical, mechanical and electronic parts of the imaging device, and therefore an internal mechanical shutter is often used in the imaging device to aid image correction. This involves periodically capturing an image while the shutter is closed in order to obtain a reference image of a relatively uniform scene that can then be used for calibration.
It is common that, following the manufacturing process of such infrared imaging devices, one or more pixels in the pixel array are declared to be non-operational at the end of the manufacturer's initial calibration phase. Such pixels are generally known in the art as “bad pixels”, and they are identified in an operability map stored by the imaging device. The pixel values generated by bad pixels cannot usually be relied upon, and therefore their pixel values are replaced by a value generated based on neighboring pixels in the image.
Moreover, it has been found that, during the lifetime of such imaging devices, the signal behaviour of one or more initially operational pixels may no longer be acceptably described by their initial calibration parameters. This may stem from various physical modifications or even mechanical damage caused by tiny internal moving particles left or released in the sensor package for example. These pixels will be referred to herein as spurious pixels. Such pixels are not listed in the initial operability map, and they can degrade the image quality.
In the case of shutter equipped imaging devices, the French patent application published as FR3009388 discloses a method of identifying such spurious pixels during any shutter closure period, giving means for recurrent updating of the operability map.
However, there are several drawbacks of using a shutter, such as the additional weight and cost, and the fragility of this component. Furthermore, for certain applications, the use of a shutter is unacceptable due to the time that is lost while the shutter is closed and calibration takes place. During this calibration period, no image of the scene can be captured.
In a shutter-less imaging device, there is a technical difficulty in identifying such spurious pixels from the image scene, particularly if the pixel values are in a textured zone of a captured image.
Assuming that spurious pixels can be identified, such spurious pixels could be merely added to the list of bad pixels. However, if the imaging device receives for example multiple shocks during its lifetime, at such a point where the density of spurious pixels in the image may no longer remain negligible, an image degradation would result.
There is thus a need in the art, in particular for shutter-less infrared imaging, for a device and method for detecting spurious pixels, at least for updating the operability map, but also for recalibrating the particular spurious pixels which would have become miscalibrated.